Whilst compression effects dynamic range, equalisation (EQ) controls frequency range. The frequency range of sound is shown in the chart below and is divided up into four bands.
The scale along the bottom of the chart shows the frequencies from 16Hz to above 16kHz. When engineers talk about the high mids they are referring to the frequency range from 1kHz to 8Khz, roughly.
The following drawing shows a typical EQ Peak Curve based around a Centre Frequency.
The centre frequency is around 750Hz and the Gain Increase (boost or cut) is around 18db. The Q Factor is the width of the frequencies effected by the boost and is measured in octaves. A high Q is narrow and a low Q is wide.
The following is is a drawing of a Shelf Curve where the frequencies above or below the centre frequency are all boosted or cut.
There are two kinds of equalisers, Parametric and Graphic and each can control a number of bands.
Here is a drawing of a typical 10 band graphic equaliser.
You will note that there are slider controls for each frequency and the scale along the base shows which frequency. The scale along the top states how many db change has been made at each frequency and it can be positive or negative (boost or cut). A typical graphic equaliser does not have any controls over the Q factor of each boost, it is normally pre-set.
For an equaliser to be called a parametric equaliser it must have a variable Q factor and a variable centre frequency. Below is an example of a parametric equaliser:
The left unit is a typical high end console analogue equaliser whereas the right one is a new generation computer program digital ones. The left one has a switchable peak/shelf High frequency control. It has two sweepable mid bands with variable Q and a peak/shelf low frequency control. The computer version has 4 Bands each with it's own centre frequency, Q width and gain. The resultant EQ curve is displayed as well. (It's a digital EQ) The mid bands of the analogue version are usually divided into two sweepable bands the the low - mid covering 100Hz - 4Khz with the other covering 600Hz - 15Khz (typically - it varies from console to console) You will note that the digital unit is sweepable from 20Hz to 20KHz in all bands.